1. Field of the Invention
The present invention relates to a plasma processing apparatus having a pair of opposing electrodes.
2. Description of the Related Art
Various conventional plasma processing apparatuses have been used to surface-treat a semiconductor wafer (to be referred to as a wafer hereinafter) in a variety of applications such as a semiconductor manufacturing process. Of these apparatuses, a so-called parallel plate type plasma processing apparatus is excellent in uniformity and has advantages capable of processing a large-diameter wafer. In addition, the apparatus arrangement is relatively simple, and therefore the apparatus of this type is very popular.
In the conventional general parallel plate type plasma processing apparatus, upper and lower electrodes oppose each other at a predetermined distance so as to be parallel to each other in a processing chamber. A wafer serving as an object to be processed is placed on, e.g., the lower electrode. For example, in an etching process, an etching gas is supplied to the chamber, and at the same time, an RF power is applied to at least one of the opposing electrodes to generate a plasma therebetween. The wafer is etched using etchant ions produced by dissociation of the etching gas. Advanced micropatterning and an increase in process speed are demanded along with an increase in integration density of semiconductor devices. To meet these demands, the density of the plasma generated between the electrodes must also be increased.
Regarding this point, a magnetron plasma processing apparatus using a magnetron as a new plasma generating method is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 57-159026 entitled "Dry Etching Method". An arrangement using a common anode electrode such as a grid-like electrode between upper and lower electrodes, in addition to normal electrodes is disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 58-12346 entitled "Plasma Etching Apparatus".
Although the magnetron plasma processing apparatus can generate a high-density plasma in a relatively high vacuum, a change in magnetic field is slower than a change in frequency of an RF electric field. A plasma state changes with variations in magnetic field. This change causes variations in energy and directivity of ions. Elements formed on a wafer may be damaged, or the shape of a pattern on the wafer may be degraded.
Although the common anode arrangement has an advantage in that an ion energy and a current density can be independently controlled, the plasma is diffused through the grid, and the density of an ion current incident on the wafer is lowered. As a result, the process rate may be lowered or the process may become nonuniform.
When an RF high-vacuum atmosphere is required along with an increase in patterning density, an impedance between an electrode and the inner wall of the chamber is lowered, and the plasma tends to be diffused.
When the plasma is diffused outward in the chamber, as described above, not only the decrease in plasma density occurs, but also metal contamination on the inner wall of the chamber occurs. For this reason, the wafer serving as an object to be processed is contaminated. This tendency will become more conspicuous in a future plasma process in a high degree of vacuum required for the advanced micropatterning.